Five-group zoom lenses are described, for example, in Japanese Laid-Open Patent Applications H10-268193, 2000-292701, and 2001-4919. These publications disclose arrangements of zoom lenses with five lens groups, including a negative lens group that is the first lens group from the enlarging side of the zoom lens. This negative first lens group moves along the optical axis for focusing and is stationary during zooming. These zoom lenses further include, from the enlarging side, a positive second lens group, a positive third lens group, and a negative fourth lens group that are moved along the optical axis relative to the negative first lens group and relative to one another to provide continuous zooming and correction for shift of the image plane caused by the continuous zooming. These zoom lenses also include a fifth lens group from the enlarging side that is a positive lens group and that is stationary during zooming. These zoom lenses satisfy various conditions.
Zoom lenses are demanded that provide a bright image in a liquid crystal projector and that are compact when used in such a projector. Recently, in addition to these requirements, there is a demand that the projection distance be short so that a zoom projection lens with a wider field angle is needed. Additionally, the zoom lenses for liquid crystal projectors need to be telecentric, or nearly telecentric, on the reducing side and have a large enough back focus to provide room for a color synthesizing system between the zoom lens and the plane of the liquid crystal display. Similar considerations apply when such a zoom lens is used in a camera using an image detecting element, such as a charge coupled device (CCD) or an image pickup tube, that uses a color separation system.
The above mentioned Japanese publications satisfy these demands to some extent, but they do not fully satisfy all the demands. For example, the zoom lenses described in Japanese Laid-Open Patent Application H10-268193 have been considered as achieving compactness with the desired image reduction on the reducing side, a proper back focus amount, and a roughly telecentric property on the reducing side. However, the f-number is about 2.5. Favorable correction of aberrations is possible with that f-number, but becomes problematic if the f-number of the zoom lens is reduced.
Zoom lenses as described in Japanese Laid-Open Patent Application 2000-292701 have small f-numbers so as to provide a bright image, but a high zoom ratio of these zoom lenses has not been achieved. Additionally, the negative refractive power of the focusing lens group is strong, which makes the diameter of the second lens group increase, as well as tends to make the diameters of the other moving lens groups increase, thereby making it difficult to make the zoom lens compact.
Zoom lenses as described in Japanese Laid-Open Patent Application 2001-4919 achieve a bright image at the wide-angle end, but it is hard to obtain a bright image at the telephoto end. Additionally, the refractive power of the fifth lens group from the enlarging side is small, which tends to increase the length of the zoom lens. If the length of the zoom lens is made to be shorter, the refractive powers of the first four lens groups from the enlarging side become so strong that a sufficient back focus is not obtained.
Therefore, in general, the prior art shows that it is not easy to design a zoom lens that satisfies the above requirements and provides a bright image (small f-number), a wide field angle, and a large zoom ratio while also maintaining compactness.
The present invention relates to a zoom lens that favorably corrects aberrations, provides a bright image, has a wide field angle and a large zoom ratio. The invention further relates to a zoom lens for a liquid crystal projection display device or to a zoom lens for a camera using an image detecting element, such as a CCD, an image pickup tube, or photographic film.